Kinetic Parameters of Gold Dissolution in Cyanide Solutions: The Effect of Complex Cyanide Ions of Mercury

Abstract

The kinetics of the gold dissolution in cyanide solutions is studied at constant values of the coverage (θ) of the gold surface by mercury atoms. The constancy of θ is ensured by maintaining an identical value of the duration (Δt) of contact of electrode with solution (after renewing its surface by cutting off a thin surface layer of metal) at a potential of –1.3 V, at which the discharge of mercury ions is limited by their diffusion to the electrode. At Δt = const kinetic dependences of the gold dissolution process correspond to the Tafel equation. Effective values of exchange current i ₀, transfer coefficient α, and reaction order by cyanide ions P are determined. With increasing value of θ their magnitude increases from values 10^–5 A cm^–2, 0.1, and 0.17 that are characteristic of purely cyanide solutions (composition 0.1 M KCN, 0.1 M KOH, and 0.01 M KAu(CN)₂) to i ₀ ≅ 2 × 10^–4 A cm^–2, α = 0.46, and P ≅ 1 at Δt = 270 s. These results are compared with the data obtained earlier during similar investigations in solutions containing thallium, lead, and bismuth. Common and individual features in the behavior of mercury-containing electrolytes are revealed. It is shown that the possible mechanism of the acceleration of the gold dissolution process in the presence of catalytically active atoms, which had been proposed in these works, may be used also for explaining the action of mercury atoms on this process.